1. Field of Invention
The present invention relates to the field of seismic data acquisition systems and methods of using same. More specifically, the invention relates to systems and methods for marine, land, and seabed seismic data acquisition in which the clock source is selected to combine advantages of distributed clock signals and clocks that are phase-locked to another clock.
2. Related Art
Seismic acquisition aims to capture the acoustic and elastic energy that has propagated through the subsurface. The structure in the subsurface may be characterized by physical parameters such as density, compressibility, and porosity. A change in the value of these parameters is referred to as an acoustic or elastic contrast and may be indicative of a change in subsurface layers, which may contain hydrocarbons. When an acoustic or elastic wave encounters an acoustic or elastic contrast, some part of the waves will be reflected back to the surface and another part of the wave will be transmitted into deeper parts of the subsurface. The elastic waves that reach the land surface or seabed surface may be measured by motion sensors (measuring displacement, velocity, or acceleration, such as geophones, accelerometers, and the like) located on the land or seabed. Acoustic waves in marine towed streamer seismic applications may be sensed by hydrophones in the streamers, or by so-called multicomponent streamers having both hydrophones and geophones or accelerometers. The measurement of elastic waves at the land or seabed surface, and acoustic waves in towed streamer seismic applications may be used to create a detailed image of the subsurface including a quantitative evaluation of the physical properties such as density, compressibility, porosity, etc. This is achieved by appropriate processing of the seismic data.
Seismic sampling in a typical seismic sensor network (whether wired or wireless) may comprise up to tens of thousands or more seismic sensors measuring the seismic vibrations or acoustic waves for oil and gas exploration. While data redundancy is increased by increasing the number of sensors, electronic noise also increases, potentially distorting the quality of the sensed data, and negatively affecting gains achieved through sensor redundancy. As illustrated schematically in FIG. 3, phase noise is added when master clock 30 signals through the sensor network in series to seismic sensor nodes 32, 34, 36, etc., each seismic sensor node having a slave clock. In order to increase perfoimance and reduce noise, slave clocks may be phase-synchronized by an electronic phase-locked loop, however this adds to cost and complexity of the systems, increases power requirements, and increases risk of system failure. An alternative is to employ expensive low-noise slave clocks. Thus, while systems and methods as illustrated schematically in FIG. 3 have enjoyed some success, there remains room for improvement. It is of utmost importancy in seismic acquisition to phase-synchronize the sampling of all the seismic sensing nodes with a minimum of noise. However, presently known systems and methods are either more expensive and less flexible, and/or generate significant noise due to the above-mentioned drawbacks. Seismic methods and systems combining advantages of both distributed clocks and phase locked clocks would be desirable to optimize power consumption and clock quality. The present invention is devoted to addressing one or more of these needs.
Published U.S. Pat. App. No. 20070070808 describes a method for recording seismic data under water comprising providing a self-contained, seismic data collection system having at least one geophone, a slave clock and a seismic data recorder; providing a master clock; synchronizing the slave clock with the master clock prior to deploying the seismic data collection system in the water. This reference, however, does not describe seismic methods and systems combining advantages of both clock distribution and phase-locked loops to optimize power consumption and clock quality.
Published U.S. Pat. App. No. 20050047275 describes a network distributed seismic data acquisition system comprising seismic receivers connected to remote acquisition modules, receiver lines, line tap units, base lines, central recording system and a seismic source event generation unit. A GPS may be used to synchronize high precision clocks as well as to provide positioning information. A master clock is designated and one or more high precision clocks is added to the network to correct for timing uncertainty associated with propagation of commands through the network. This reference, however, does not describe seismic methods and systems combining advantages of both clock distribution and phase-locked loops to optimize power consumption and clock quality.
Published U.S. Pat. App. No. 20020063588 describes techniques for providing clocking signals for use in checking seismic equipment. Clocks on and off an integrated circuit chip are aligned so that clocks on the chip are synchronized to one of the rising and falling edges of a master clock and those off the chip are synchronized to the other of the rising and falling edges of the master clock. This apparently permits a certain ease of interfacing circuits controlled by those clocks. Programmable clocks on the chip can be reprogrammed during operation to conserve power. This reference, however, does not describe seismic methods and systems combining advantages of both clock distribution and PLL to optimize power consumption and clock quality.